X-ray tube protective circuit including thermionic discharge means connected in series with the x-ray tube

ABSTRACT

A POWER SUPPLY CIRCUIT FOR OPERATING AN X-RAY TUBE INCLUDES A DC CONSTANT POTENTIAL SOURCE PROVIDED WITH STORAGE CAPACITOR MEANS FOR DISCHARGING CURRENT THROUGH THE X-RAY TUBE WHEN THE X-RAY TUBE IS SWITCHED INTO OPERATION. A PAIR OF THERMIONIC DIODES ARE CONNECTED IN SERIES BETWEEN EACH PRINCIPAL ELECTRODE OF THE X-RAY TUBE AND THE AFOREMENTIONED CAPACITOR MEANS. THE LEVEL OF EMISSION IN EACH THERMIONIC DIODE IS LIMITED TO COUPLE SUBSTANTIALLY NORMAL CURRENT TO THE X-RAY TUBE WHEN THE X-RAY TUBE IS SWITCHED INTO OPERATION, WITHOUT THE OCCURRENCE OF AN EXCESSIVE VOLTAGE DROP ACROSS THE DIODE. HOWEVER, THE LEVEL OF EMISSION IS LIMITED SO THAT A SUBSTANTIALLY INCREASED VOLTAGE DROP TAKES PLACE ACROSS EACH DIODE IN THE EVENT OF SPURIOUS DISCHARGE OR SHORT CIRCUIT IN THE TUBE SUCH THAT CURRENT SUBSTANTIALLY IN EXCESS OF THE NORMAL CURRENT TENDS TO BE DRAWN.

United States Patent [72] Inventor Robert M. Gager Elmhurst,1ll. [21} Appl Nov 722,881 [22] Filed Apr. 22,1968 [45] Patented June 28,1971 [73] Assignee Picker Corporation White Plains. NJ.

[54] X-RAY TUBE PROTECTIVE CIRCUIT INCLUDING THERMIONIC DISCHARGE MEANS CONNECTED IN SERIES WITH THE X-RAY TUBE 12 Claims, 2 Drawing Figs.

[52] U.S.Cl 250/103, 3l5/3ll,317/20,3l7/51 [SI] Int. Cl G03b 41/16, v HOlj 37/22 [50] Field of Search 250/97, 98, 100, I02, l03;317/51,20;3I5/31l;323/4 [56] References Cited UNITED STATES PATENTS 2.222536 11/1940 Kuntke et al. 250/97 069.548 l2/l962 Bavoretal. 3.l87,224 6/l965 Massenat Primary Examiner.lames W. Lawrence Assistan! Examiner A. L. Birch Attorney-Buckhorn, Blore, Klarquist & Sparkman mionic diode is limited to couple substantially normal current to the X-ray tube when the X-ray tube is switched into operation, without the occurrence of an excessive voltage drop across the diode. However, the level of emission is limited so that a substantially increased voltage drop takes place across each diode in the event of spurious discharge or short circuit in the tube such that current substantially in excess of the normal current tends to be drawn.

PATENTEUJUN28|97I 3588.510

M I I 12 I 62 I I 68 70 I 69 I 48 I um I 64-'- =-66 I O. O 20 [I O 0 200 400 600 800 I000 I200 I400 I600 I600 CURRENT, ROBERT M. GAGER INVENTQR BY BUCKHORN, BLORE, KLARQUIST & SPARKMAN I ATTORNEYS BACKGROUND OF THE INVENTION A constant potential DC power supply for an X-ray tube can be of advantage for a number of reasons, particularly when a grid-controlled X-ray tube is employed. A constant supply permits the X-ray tube to be operated at substantially any time for a very short period of time and does not require synchronization .of grid pulses with an AC line frequency. A constant potential system is also relatively cheap and simple compared to the other alternatives such as complex threephase generator circuits. Constant potential generators usually include output storage capacitor means which are discharged through the X-ray tube as the X-ray tube is pulsed into operation. Constant potential generators traditionally have employed series resistance to limit the current in the event of a spurious discharge or short circuit, but this method is practical only for relatively low X-ray tube currents. A typical value of current-limiting resistance is 1 ohm per volt, which limits current to a maximum of one ampere. Thus, for a 150 kilovolt tube, 150,000 ohms of resistance might be included in series therewith. Then, should the X-ray tube become effectively shorted, only one ampere of current would be drawn through the tube from the supply. Assuming the X- ray tube draws a low current, for example milliamperes, during normal operation, only 1500 volts is normally dropped by the protective resistance, leaving 148,500 volts for application to the tube. However, for a tube normally drawing a much higher current, e.g. 500 milliamperes, the voltage across the protective resistance would be half the generated voltage. Protective series resistance in the latter case is obviously impractical. But without the protective resistance, stored energy in the capacitor means is capable of seriously damaging or destroying the X-ray tube or causing other damage in case of spurious discharge in the tube.

SUMMARY OF THE INVENTION According to the present invention, a protective power supply circuit for operating an X-ray tube comprises a substantially constant potential source, and at least one thermionic discharge device coupled in series between the source and the Xray tube. The electron emission in the discharge device is limited to couple substantially normal current to the X-ray tube when the X-ray tube is pulsed into normal operation, withoutexcessive voltage drop occuring across the discharge device. The level of emission is limited so that a substantially increased voltage drop takes place across the discharge device in the event the X-ray tube draws current in excess of normal operating current. Thus, the discharge device is operated substantially at or near saturation, and a great excess of current will not be coupled therethrough. When the X-ray tube draws normal current, the discharge device is characterized by a first and rather low value of resistance, but in the event of spurious discharge or breakdown, and excessive current drawn, the discharge device is characterized by an appreciably higher and increasing resistance such that a much greater voltage drop occurs thereacross. As a result, the current through, and the voltage across, the X-ray tube is limited to reduce the possibility of damage.

It is accordingly an object of the present invention to provide an improved supply circuit for preventing X-ray tube damage in the event of spurious discharge or the like.

It is a further object of the present invention to provide an improved X-ray tube supply circuit wherein the X-ray tube current is limited to predetermined values.

It is a further object of the present invention to provide an improved protective power supply circuit for operating an X- ray tube wherein an increasing value of resistance is inserted in series with the Z-ray tube when excessive current is drawn.

anode.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings.

DRAWINGS DETAILED DESCRIPTION Referring to FIG. 1, a protective power supply circuit 10 for X-ray tube 12 includes an input transformer 14 having a primary winding 16 for connection to AC supply lines, and a high voltage secondary winding 18 having a center tap connection 20 which is grounded. The remaining end terminals 22 and 24 of the secondary winding are connected across a bridge rectifier comprising diodes 26, 28, 30, and 32 poled to provide a rectified output at bridge rectifier output terminal 34, positive at terminal 34 relative to bridge rectifier terminal 36. Diodes 26, 28, 30, and 32 may comprise high vacuum valves or solidstate devices. Since the high voltage secondary winding 18 is grounded at its center tap, terminal 36 is negative with respect to ground. Storage capacitor means, including a first storage or filter capacitor 38 and a second storage or filter capacitor 40 in series, is disposed across terminals 34 and 36. The center interconnection between capacitors 38 and 40 is grounded. Capacitors 38 and 40 together with the input transformer and the bridge rectifier form a substantially constant potential source of power for the X-ray tube 12. The bridge comprising diodes 26, 28, 30, and 32 rectifies the high voltage AC output of the transformer, and the rectified voltage charges capacitors 38 and 40 to a relatively high and constant DC value. When the X-ray tube 12 is operated or turned on, the current therefor is substantially immediately delivered from capacitors 38 and 40.

According to the present invention, first and second thermionic discharge devices 42 and 44 are interposed between the ungrounded terminals of capacitors 38 and 40 and X-ray tube supply terminals 46 and 48, respectively. These discharge devices suitably comprise high vacuum thermionic valves. Discharge device 42, for example, includes an anode 50 connected to terminal 34 and a filament 52, one side of which is connected to terminal 46. Filament 52 is connected across the secondary winding of a filament transformer 54, the primary of which is coupled to a suitable source of power as hereinafter more fully described. Discharge device 44 includes an anode 56 coupled to terminal 48 and a filament 58, one side of which is connected to terminal 36. Filament 58 is connected across the secondary of the filament transformer 60, the primary of which is coupled to a suitable source of power as will hereinafter be more fully described.

X-ray tube supply terminals 46 and 48 are suitably connected respectively to principal electrodes of the X-ray tube, i.e. to the X-ray tube anode electrode and to a filament electrode. Terminal 46 is connected to 'X-ray tube anode 62, while terminal 48 here comprises a common connection between two secondaries of X-ray tube filament transformers 64 and 66, suitably included in protective power supply circuit 10. Terminal 48 is connected to a center connection between X- ray tube filaments 68 and 69, while the remaining secondary terminals of transformers 64 and 66 are connected to the remaining filament terminals. The primaries of transformers 64 and 66 are selectively energized from a suitable source of power. The filaments 68, 69 are suitably operated alternatively for providing different sized focal spots on the X-ray tube X-ray'tube 12 is also desirably provided with a gate means or grid switching means for switching the X-ray tube into operation. This gate means is nonnally electrically biased (by means not shown) to prevent discharge in the X-ray tube, and the gate means is here schematically illustrated as grid 70. When an appropriate pulse is applied to grid 70, normal current flow discharge occurs at the X-ray tube, with X-rays being produced at anode 62. Although the gate means is here illustrated as a part of a grid controlled X-ray tube, it is understood that an additional control tube for the same purpose may be connected in series with the X-ray tube if so desired.

it is noted that discharge devices 42 and 44 are essentially in series with the principal current carrying path of the X-ray tube 12 and are poled in the same direction to carry the X-ray tube current. That is, the filament of device 42 is connected to 15 the anode of X-ray tube 12, and The X-ray tube filaments are connected to the anode of device 44. It will be observed that since the present circuit is a DC circuit, no inverse voltage appears across either device 42 or 44, or X-ray tube 12.

According to the present invention, the current supplied filaments 52 and 58 of discharge devices 42 and 44 is predetermined such that electron emission in devices 42 and 44 is limited to a valuecapable of supplying current only in the range of normal X-ray tube current. This current is, of course, supplied as the X-ray tube is switched into operation by application of a pulse at grid 70. The level of emission in discharge devices 42 and 44 is limited such that in the event of spurious discharge or short .circuiting between the principal electrodes of X-ray tube 12, e.g. between anode 62 and one of the filaments, a substantially increased voltage drop takes place across each discharge device. The filament current in discharge devices 42 and 44 is preselected by adjusting the primary currents supplied transformers 54 and 64, so that the discharge devices operate near saturation when carrying normal X-ray tube current. In the event of substantial short circuiting in the X-ray tube, a path for such extra current as might result in physical damage to the tube or other components, will not be available. As the X-ray tube starts to draw extra current well above its normal current value, discharge devices 42 and 44, which normally drop a relatively low voltage, will now drop a considerably higher proportion of the supply voltage, thereby protecting tube 12. Discharge devices 42 and 44 will drop substantially the entire supply voltage if the X-ray tube is shorted out. The stored energy in capacitors 38 and 40, which represents a potentially destructive force, will not be destructively dissipated through the X-ray tube, but will be gradually and harmlessly dissipated through the increased resistanceof discharge devices 42 and 44.

The FIG. 2 plot is a somewhat idealized characteristic curve for a thermionic diode valve, for example, for discharge device 42 or discharge device 44, plotting voltage drop versus current. It will be seen that for low current, e.g. in the vicinity of normal operating current for X-ray tube 12, in this case under approximately 1,000 milliamperes, the resistor represented by one valve is approximately 2,500 ohms. At 500 milliamperes operating current, and at a voltage of 150 KV, two valves will drop a total voltage of approximately 2,500 volts, leaving 147,500 volts across an X-ray tube. However, should the current rise to approximately 1,500 milliamperes, the resistance of each valve increases so that a drop of approximately 75 kilovolts occurs across each valve, thereby dropping the entire supply voltage of 150 KV without damage to the tube or other components. Therefore, the circuit according to the present invention provides ample protective, in a constant potential supply, for normally supplying high X-ray tube currents, without excessive voltage drop being encountered when normal current are drawn. The current flowing in the circuit, even under short circuit conditions, is in any event limited to the maximum value which electron emission in discharge devices 42 and 44 will couple.

Although a pair of discharge devices are herein illustrated as inserted between the storage capacitor means and the X-ray tube, and although the protection afforded by two valves is desirable, it will be understood that only one valve may be included if so desired. For example, one such valve may be serially included between the ungrounded end of capacitor 38 and terminal 46 connected to X-ray tube anode 62.

While I have shown and described a preferred embodiment of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. 1 therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

lclaim: v

l. A protective power supply circuit for operating an X-ray tube comprising:

a substantially constant potential source coupled for selectively providing a predetermined operating current to said X-ray tube,

and at least one thermionic discharge device coupled in series with said X-ray tube between a principal electrode of said X-ray tube and said substantially constant potential source, said discharge device being provided with means for establishing the electron emission therein at a predetermined value to couple substantially normal current to said X-ray tube during normal operation thereof without excessive voltage drop across said discharge device,

the level of said emission being limited by said last mentioned means so that a substantially increased voltage drop takes place across said discharge device in the event said X-ray tube draws current substantially in excess of normal operating current.

2. The circuit according to claim 1 wherein said discharge device comprises a high vacuum valve, and wherein said means for establishing said emission comprises a filament in said valve and power supply means for said filament, said power supply means providing a current to said filament for determining a limited emission level in said valve.

3. The circuit according to claim 1 including a said thermionic discharge device in series with each of the electrodes of said X-ray tube and said source. f

4. The circuit according to claim 1 wherein said source includes a DC power supply and storage capacitor means for providing current to said X-ray tube when said X-ray tube is operated, said discharge device being serially interposed between said storage capacitor means and an electrode of said X-ray tube. 3

5. The circuit according to claim'l wherein said X-ray tube comprises a grid controlled tube having a grid switching means for controlling discharge in said X-ray tube.

6. The circuit according to claim 1 wherein said thermionic discharge device is operated at a level of emission providing a voltage versus current characteristic defining a first and substantially low value of resistance for current supplied said X- ray tube during normal operation thereof, and defining a higher value of resistance for currents materially above the current normally supplied said X-ray tube during normal operation.

7. A protective power supply circuit for operating an X-ray tube comprising:

a substantially constant potential source coupled for selectively providing. a predetermined operating current to said X-ray tube, said source including a pair of storage capacitors each having one terminal grounded and each having a remaining terminal for supplying current to said X-ray tube when said X-ray tube is operated,

and a pair of thermionic discharge devices coupled respectively between ungrounded terminals of each said storage capacitor and a separate principal electrode of said X-ray tube, each said discharge device comprising a thermionic diode the electrodes of which are poled to couple current in the same current direction as the electrodes of said X- ray tube,

the electron emission in at least one of said discharge r devices being limited to couple substantially normal current to said X-ray tube during normal operation thereof without excessive voltage drop across the said one of the discharge devices, the level of said emission being limited so that a substantially increased voltage drop takes place across the last mentioned discharge device in the event said X-ray tube draws current substantially in excess of normal operating current.

8. A protective power supply circuit for operating an X-ray tube comprising:

an input transformer adapted to be powered by a source of alternating current, said transformer having a high voltage secondary winding,

a bridge rectifier intercoupled with the high voltage secondary of said transformer for providing a high DC voltage across bridge rectifier output terminals,

storage capacitor means coupled across the said bridge rectifier output terminals,

a pair of X-ray tube supply terminals to which principal electrodes of said X-ray tube may be connected,

a first thermionic diode coupled between a bridge rectifier output terminal and a first X-ray tube supply terminal, the electrodes of said diode being poled to pass current in a first direction relative to the said first X-ray tube supply terminal,

a second thermionic diode coupled between a second bridge rectifier output terminal and a second X-ray tube supply terminal, the electrodes of the last mentioned diode being poled to pass current in a second direction relative to the second X-ray tube supply terminal,

and gate means for switching said X-ray tube into operation,

causing current flow in said X-ray tube,

the electron emission in said thermionic diodes being limited to couple a first value of current to said X-ray tube, without excessive voltage drop occurring across said diodes, when said X-ray tube is switched into normal operation, the level of said emission being limited so that a substantially increased voltage drop takes place across said diodes for protecting said X-ray tube in the event said X-raY tube draws current appreciably in excess of said first value.

9. The circuit according to claim 8 wherein each said thermionic diode is provided with a filament for determining the electron emission therein, and a filament supply transformer to which a predetermined current is provided for determining the limited emission level in the respective diode.

10. The circuit according to claim 9 wherein the high voltage secondary winding of said transformer is provided with a grounded center tap connection,

and wherein said storage capacitor means comprises first and second storage capacitors connected in series, with the interconnection therebetween being grounded.

11. The circuit according to claim 10 wherein the X-ray tube comprises a grid controlled tube having a grid switching means for controlling discharge in said X-ray tube.

12. The circuit according to claim 8 wherein each said thermionic diode is operated at a level of emission providing a voltage versus current characteristic defining a first and substantially low value of resistance for current supplied said X- ray tube during normal operation thereof, and defining a higher and increasing value of resistance for currents materially above the current normally supplied said X-ray tube during normal operation. 

